This invention is directed to a novel vibration isolator. More particularly, the present invention is directed to a hybrid elastomeric-and-metal spring vibration isolator with a low natural frequency at its rated load.
While this isolator may be considered for any of a number of general applications, it has particular advantages in mounting large diesel engines in such a way as to minimize/eliminate transmission of low frequency vibrations of the engine to its support structure. Specific applications of such diesel engines include, but are not limited to, locomotive engines and diesel engines on fishing boats.
It is the current practice in mounting large diesel engines, which, in some cases, with associated driven gear units, generators and auxiliary equipment, can weigh up to thirty tons, to produce a support deck that has positive camber (i.e., is bowed upwardly in the middle). This is to offset the vertical deflection that takes place when the engine assembly is positioned on them. This positive camber significantly complicates the preparation of support pads for the engine assembly. The platform must be deflected to its final curvature before the pads can be machined. Equipment and tools to facilitate such mounting pad preparation are heavy, complex and expensive. Further, the process using this equipment is time consuming and labor intensive, requiring as much as several days time to complete.
Some installations use elastomeric mounts for the engine assembly with natural frequencies of 7 Hz, or higher. Since large diesel engines have operational speeds in the 200-2000 rpm range, idle excitations will generally fall in the 6-10 Hz range. Since the natural frequency of the mounting system lies in the range of idle speed excitations for such engines, excitation of the suspension at its natural frequency (resonance) can occur when the diesel engine is at idle. Resonant operation does not merely result in full transmission of the engine's vibration to the support, as would result from hard mounting the engine directly thereto but, rather, can actually amplify the vibrational excitation up to ten times the response level of a hard mounted engine assembly. Obviously, such amplification can have undesirable effects and it is an important design consideration to try to move the natural frequency of the system outside the operating frequency of the engine or, if that is not possible, to a frequency (e.g., 3-5 Hz) through which the engine quickly passes during startup to prevent the build up of destructive harmonic vibrations.
Elastomeric isolators which could produce natural frequencies in the 3-5 Hz range have high static deflection and associated creep and drift that makes them unsuitable for some diesel engine applications where only minor (one inch or less) relative displacement can be tolerated due to connections to associated upstream and downstream hardware (e.g., air inlet ducts, exhaust ducts, fuel lines, auxiliary electrical power connections, compressed air lines). Further exacerbating the problem is the fact that the manner in which these conventional elastomeric blocks are mounted on the platform and attached to the engine assembly makes it difficult to focalize or semi-focalize the mount. Without some form of focalization in a low frequency suspension, the lateral translational and rotational modes of response will normally be coupled (due to the positioning of the mounts well below the center of gravity of the assembly) resulting in even greater undesired rocking motion of the engine.
The hybrid elastomer-and-metal spring vibration isolator of the present invention provides an isolation system with a low natural frequency (e.g., 3-5 Hz). This isolator comprises an elastomeric sandwich mount having a large elastomeric section bonded to first lower and second upper metal plates. The elastomeric section has a plurality of cored out pockets that receive helical metal springs. The upper and lower plates have cavities molded into them, the cavities extending into the cored out pockets through the center of the springs. The protruding end surface of the cavities are coated with a layer of elastomer which serves to provide cushioned snubbing in the compressive direction. The coil springs react against the nether (i.e., underneath) surfaces of the upper and lower plates, with a protective bearing seat being provided for the upper plate to protect the elastomer bonded thereto. The coil springs are designed with initial preload to fully support the static weight of the supported device so the elastomer experiences only dynamic loading. This significantly extends the service life of the isolator by reducing dynamic stress/strain levels on the elastomer.
A tension bolt with a length that exceeds the normal distance between the cavities of the upper and lower plates is equipped with a pair of load-transferring metal washers which engage elastomer-coated snubbing surfaces which, in turn, serve to provide cushioned snubbing of relative movement between upper and lower plates of the sandwich mount in the expansion direction. In addition, the interior coated surfaces reduce sound from metal-to-metal contact that would otherwise result between the tension bolt and the bare surfaces of the cavities. This tension bolt and its associated securement nut provide a first level of coil spring precompression restraint for the isolator, as well as limit the extension relative deflection, and hold the assembly together. A special retaining nut which engages a protruding end of the tension bolt preloads the metal springs to a second higher level of precompression which is substantially equal to the static load carried by the isolator. This retaining nut is designed to be removed prior to installation of the engine on the isolator platform.
Various other features, advantages and characteristics of the present invention will become apparent after a reading of the following specification.